The neuronal cytoskeleton is a dynamic, highly specialized set of structures that plays critical roles in many aspects of the nervous system, ranging from generation and maintenance of neuronal morphologies to defining functional domains of a neuron. To serve these functions, the components of the cytoskeleton must be biochemically specialized to control organization and stability. The experiments in the application address both the nature of functional specialization for the neuronal cytoskeleton and the cellular processes that affect them. These are genetic and biochemical adaptations of the cytoskeletal elements to specific biological requirements of neurons. Some result from programs initiated during differentiation of neurons and glia, while others represent responses to the local environment and are sensitive to subsequent changes in that environment. A novel biochemical specialization of the neuronal microtubule cytoskeleton has been identified that stabilizes axonal microtubules. The experiments in the first aim analyze the biochemistry of cold insoluble axonal tubulin and define physiological roles for stable axonal microtubule segments in neuronal function. The large size of many axons requires that the axonal cytoskeleton be influenced by the local microenvironment. Work in the last funding period on mutant strains of mice with defective myelination established that the myelinating glia profoundly influence both the composition and the local properties of the axonal cytoskeleton. The ext4nt to which the glial microenvironment can alter the organization and dynamics of the underlying axonal cytoskeleton will be continued to be examined in demyelinate and myelinated nerves. Experiments under aim 2 seek to define metabolic pathways for local modulation of the axonal cytoskeleton by the glial environment. The interaction between myelinating glia and axons in the PNS and CNS will be further characterized to determine the extent to which myelination sculpts the functional architecture of the axon. During the last funding period evidence accumulated that formation of compact myelin in the CNS was required for the maturation of the neuronal cytoskeleton. Experiments in aim 3 will identify pathways to myelinating glia modulate neuronal gene expression. These experiments will help identify mechanisms by which a specific molecular response of the axon to its environment is generated. The goal is to understand dynamics of the neuronal cytoskeleton that play critical roles in development, regeneration, and neuropathology.